1. Field of the Invention
The invention relates to a process for the preparation of the sugar alcohols mentioned above in the title, in epimer-free form, from the corresponding sugars xylose, .alpha.-D-glucose, 4-O-.beta.-D-galactopyranosyl-.alpha.-D-sorbitol or 4O-.alpha.-D-glucopyranosyl-.alpha.-D-glucopyranose by continuous catalytic hydrogenation using hydrogen. The course of the reaction can be illustrated by means of the following reaction schemes: ##STR1##
The four sugar alcohols which can be prepared in eipmer-free form according to the invention are known and can be described as follows:
The sweetening potency of xylitol reaches about 80-100% of the sweetening potency of sucrose. The potency can be increased by adding to the aqueous solution artificial sweeteners, for example cyclohexyl sulphamate or methyl phenylalanine-aspartate, and the product can be obtained in crystalline form by joint vacuum crystallization. However, it is also possible to mix the artifical sweeteners in solid form with the crystallized xylitol. Xylitol can also be mixed in liquid or solid form with other sweet-tasting carbohydrates, for example maltitol, lactitol, sorbitol and the like.
Owing to its pleasantly sweet taste, xylitol is suitable as a sugar replacement in diabetic diets and as a non-cariogenic sweetener in confectionery and oral pharmaceuticals. The use of xylitol in diabetic food is permitted in unlimited amounts in accordance with the dietary statute (Federal German Law Gazette I 1982, page 71).
Pure xylitol is particularly well suited for processing in confectionery such as sweets and chewing gum (Swiss Dent. I (7/8) 1980, page 25 to 27). Even products for treating the mouth and pharyngeal cavities such as toothpastes, sore throat tablets, and cough sweets are increasingly sweetened with xylitol (Swiss Dent. III (7/8) 1982, page 25 to 30).
In the human body, sorbitol is only resorbed to a minor extent and only this proportion is broken down. Sorbitol is therefore suitable as a sugar substitute for diabetics and as a low-calorie sweetener. Furthermore, it is less cariogenic than glucose or other sugars.
The potency of sorbitol reaches about 50-60% of the potency of sucrose. The potency can be increased by adding to the aqueous solution artificial sweeteners, for example cyclohexyl sulphamate or methyl phenylalanineaspartate and the product can be obtained in crystalline form by joint vacuum crystallization. However, it is also possible to mix the artificial sweeteners in solid form with the crystallized sorbitol. Sorbitol can also be mixed in liquid or solid form with other sweet-tasting carbohydrates, for example maltitol, lactitol, xylitol and the like.
So far it has not been possible to identify any toxic effects, even in long term studies (Ullmanns Encyklopadie der technischen Chemie, Volume 24, Weinheim 1983, p. 774), and therefore there are many suitable applications in the food sector in the preparation of diabetic products and of sugar-free sweets and foods having a low nutritive value.
Xylitol or sorbitol is often prepared by a discontinuous process (batch process) in which a pulverulent nickel catalyst is employed in suspension.
In the human body, lactitol is not broken down as a carbohydrate and in the small intestine is neither hydrolyzed nor absorbed. Lactitol is therefore suitable as a sugar substitute for diabetics. Moreover, it is less cariogenic than sucrose.
The potency of lactitol reaches about 40% of the potency of sucrose. The potency can be increased by adding to the aqueous solution artificial sweeteners, for example cyclohexyl sulphamate or methyl phenylalanineaspartate and the product can be obtained in crystalline form by joint vacuum crystallization. However, it is also possible to mix the artificial sweeteners in solid form with the crystallized lactitol. Lactitol can also be mixed in liquid or solid form with other sweet-tasting carbohydrates, for example fructose, sorbitol, xylitol and the like.
So far it has not been possible to detect any toxic effects, even in long term studies (Ullmanns Encyklopadie der technischen Chemie, Volume 24, Weinheim 1983, p. 779), and therefore there are many suitable applications in the food sector in the preparation of diabetic products and of sugar-free sweets and foods having a low nutritive value.
In the human body, maltitol is broken down only with difficulty by amylolytic enzymes. Maltitol is therefore suitable as a sugar substitute for a calorie-reduced diet and for diabetics (Ullmann's Encyklopadie der technischen Chemie, Volume 24, Weinheim 1983, p. 771).
The potency of maltitol reaches the potency of sucrose. Maltitol can be mixed in liquid form with other sweet-tasting carbohydrates, for example fructose, sorbitol, xylitol and the like. The use of maltitol in the beverage industry is particularly advantageous owing to its high potency and very low tendency to crystallize even at high concentrations.
2. Description of the Realted Art
EP 39,981 discloses a discontinuous process (batch process) for the preparation of lactitol, which has so far not been detected in nature, this process employing a pulverulent nickel catalyst in suspension.
U.S. Pat. No. 3,741,776 discloses a discontinuous process (batch process) for the preparation of maltitol, which has so far not been detected in nature, this process employing a pulverulent nickel catalyst in suspension.
Discontinuous processes have the disadvantage that their capacity is very small in relation to the reaction volume, and therefore large reaction apparatuses and storage tanks are necessary. The energy consumption is uneconomical and the manpower requirements are relatively high.
Part of the abovementioned disadvantages are avoided in continuous powder-catalyzed processes which function using a plurality of cascaded hydrogenation reactors. However, the troublesome manner of selectively activating, circulating and quantitatively filtering off the pulverulent catalyst from the reaction product still remains. The catalyst slurry pumps are subjected to high mechanical stress. The quantitative removal of the pulverulent catalyst is expensive (alternate arrangements of coarse and fine filtration apparatuses). Furthermore, there is a high risk that the catalyst will lose its activity relatively quickly owing to the additional operations (high catalyst consumption). It is therefore desirable that the reaction proceeds over fixed-bed catalysts which must have a high specific activity which must also be sustained for a relatively long period of 1 to several years since frequent catalyst exchange is also expensive in the case of fixed bed reactions.
It is also customary with fixed bed catalysts to connect several reactors in succession, this giving a plurality of reaction zones connected in series (DE-A-3,214,432).
Use is made of nickel catalysts on a carrier (SiO.sub.2 /Al.sub.2 O.sub.3), these catalysts having extremely high active surfaces of 140-180 m.sup.2 /g so that the catalysts are so active that they must be stabilized by additional chemical treatment methods, for example by oxygen gas absorption to form monomolecular oxygen layers on the catalyst surface (DE-A-3,110,493). However, the deactivating stabilization of the catalyst then requires such high reaction temperatures during the hydrogenation of sugars (130-180.degree. C.) that uncontrollable side reactions can occur such as discoloration by caramelization and destructive hydrogenation (hydrogenolysis) of the saccharide alcohols to the extent that methanol and even methane are produced. Moreover, with this type of reaction, heavy metals such as nickel, iron or cobalt continuously go into solution in ionic or colloidal form which necessitates on the one hand a subsequent treatment of the hydrogenated product with activated carbon and on the other hand deionization using ion exchangers.
Since the known hydrogenation processes operate on sugar solutions having a pH which has been adjusted to 7-13, basic alkali metal compounds or alkaline earth metal compounds must be added to the starting solutions and must likewise be removed from the end product with difficulty (DE-A-3,110,493; DE-A-3,214,432).
Moreover, it is expected that, under the hydrogenation conditions, significant epimerization will occur, so that, for example, D-xylose gives not only xylitol but also lyxitol (arabinitol and ribitol); .alpha.-D-glucose is expected to give not only sorbitol but also mannitol.
Furthermore, the splitting effect on the carbohydrate chain of sugars during catalytic hydrogenation on Raney nickel is known; DE-A-2,756,270 describes this effect on a sugar mixture such as is obtained from the auto-condensation of formaldehyde, a significant shift from longer carbon chain lengths to shorter carbon chain lengths being observed within the scope of the cited exemplary embodiments. According to DE-A-2,831,659, this type of splitting is not observed on precious metal catalysts of sub-group 8.
EP 152,779 discloses a process for the hydrogenation of .alpha.-D-glucopyranosido-1,6-fructose on fixed bed catalysts. However, in this process a mixture of two reduction products is obtained, in particular .alpha.-D-glucopyranosido-1,6-mannitol and .alpha.-D-glucopyranosido-1,6sorbitol in a ratio of about 1:1.
It was therefore surprising that in the present invention the sugars are not only virtually completely converted but, with the avoidance of epimerization and carbon chain splitting and also with the avoidance of the formation of higher molecular weight components by condensation reactions with ether formation, only one sugar alcohol, as described earlier in the reaction schemes, is obtained. This is of importance for the direct use of the product as a diet component without further purification.